powerpc: dts: p2020: Add fsl/p2020si-pre.dtsi and fsl/p2020si-post.dtsi symlinks
P2020 DTS files in upstream Linux kernel use fsl/p2020si-pre.dtsi and
fsl/p2020si-post.dtsi include device tree files.
Add symlinks for these include device tree files into U-Boot powerpc
directory and points them to U-Boot inline device tree files p2020.dtsi and
p2020-post.dtsi.
This allows to use P2020 DTS files from upstream Linux kernel in U-Boot.